There is considerable interest in the applications in medicine of four radioisotopes of indium and gallium, 111In, 113IN, 67Ga, and 68Ga. Complications arise in the design of radiopharmaceuticals labeled with indium(III) and gallium(III) because of the fact that both metals undergo extensive hydrolysis in aqueous systems, and form very stable chelates with the plasma protein transferrin, and with other biological chelating ligands containing negative oxygen donor atoms. Simple gallium and indium chelates of low stability are quickly converted to chelates of the natural biomolecules that are capable of carrying trivalent metal ions such as iron(III). For the highly stable synthetic chelates of gallium(III), the situation is further complicated by the fact that the rate at which equilibrium is reached is very slow compared to the rates of many biological processes. Thus gallium and indium chelates may retain their integrity for a period of time even though equilibrium favors eventual complete exchange with biomolecules such as transferrin. Further difficulties arise from a lack of data on stability constants and rate constants since studies of chelates of gallium(III) and indium(III) have generally been neglected by coordination chemists. It is planned to study the equilibrium and kinetics of formation of gallium(III) and indium(III) chelates of synthetic and natural chelating agents to provide the data necessary to design improved radiopharmaceuticals containing these metals, and to improve the understanding and predictability of their exchange reactions with biomolecules and with intracellular binding sites. It is also planned to synthesize new types of highly stable chelates of gallium(III) and indium(III) that will retain their integrity in the presence of biological iron(III) carriers such as transferrin, thus making possible the development of more highly specific and efficient radiopharmaceuticals for the diagnosis and scanning of tumorous tissue.